If a bicycle is to be used for touring or over hilly terrain, a variable speed transmission is essential. Unfortunately, none of the prior art systems performs satisfactorily.
For example, one of the most common variable speed transmissions employs a chain and a series of sprockets of different diameters. The chain can be moved by the rider from one sprocket to another to change the drive ratio. With this system, drive ratio selection is difficult to accurately select because the chain may skip over the desired sprocket. Moreover, the chain may be "thrown". At the very best this requires manual placement of the chain over one of the sprockets, and under certain circumstances, the back wheel may lock and cause damage to the bicycle and injury to the rider.
Numerous attempts have been made to overcome these disadvantages. For example, another system uses a complex planetary gear system to provide only three different drive ratios. Generally none of these other systems provides an inexpensive and reliable variable speed transmission which is capable of providing many different drive ratios.